1. Field of the invention
This invention relates to electrical test probes, and more specifically, to a high impedance, wide bandwidth probe, for use with logic analyzers, oscilloscopes, and other electronic test instruments.
2. Statement of the Problem
Electrical test probes are used to transmit a signal voltage from a circuit under test to an electronic test instrument, such as a logic analyzer. There are two desirable electrical characteristics for test probes: First, the signal response at the test instrument should be an accurate (although attenuated) representation of the probed signal over the range of frequencies of interest. Second, the probe should not influence, or "load" the output from the circuit under test. In order to provide these characteristics, a probe should exhibit both high resistance and low capacitance. A high probe resistance allows relatively little of the output current to flow through the probe, thereby minimizing any loading effect of the probe on the circuit under test. The frequency response of a probe is dependent upon the capacitance of the probe in parallel with the resistance of the circuit under test. Capacitive reactance varies as a function of frequency, causing the impedance of the probe to fall as the applied frequency increases. The effective bandwidth of prior art probes has thus been limited by probe capacitance. Minimizing the probe tip capacitance has been seen as a solution to increasing the effective bandwidth of a probe. Compensating for probe capacitance has been attempted by using active electronics in the probe tip, but with the drawback of a bulky and easily damaged probe tip, which also requires additional components as well as a source of electrical power.
Passive probes are those which do not require electrical power for their operation. Prior art passive probes typically use either standard coaxial cable with a center conductor having low DC resistance, or "lossy line" coaxial cable having a significant distributed series resistance. A probe using lossy line cable is disclosed in U.S. Pat. No. 2,883,619, to Kobbe, et al. Most passive probes can be characterized as having three basic elements: a tip network, a section of cable, and a terminating network. These elements are connected in series and, when properly designed, provide a frequency compensated attenuator with a relatively flat response across a given frequency range. For both DC and low frequency AC signals, the frequency response of the probe is primarily a function of the tip and terminating networks (plus any DC resistance contributed by the cable). However, when the risetime of the input signal applied to the probe becomes a substantial fraction of the wavelength of the probe cable, the cable becomes a transmission line of substantial electrical length terminated at both ends with an impedance other than the characteristic impedance of the line. This mis-termination causes reflection of the input signal from the ends of the cable, resulting in unwanted distortion of the signal. The reflected signal not only results in a frequency response which is far from flat, but also causes a damped oscillation effect, called "ringing," when a square wave or a fast risetime signal is applied to the probe input. It would appear that a solution to the signal reflection problem would be termination of both ends of the cable with its characteristic impedance in order to absorb such reflected energy. The impedances required, however, are so low that they either unduly load the circuit under test or result in excessive attenuation. In addition, as shorter lengths of cable are used, signal energy reflected from the mis-terminated ends of the cable results in a phenomenon called "crossover dip." The input signal is reflected from the termination and travels back toward the signal input node, and is reflected back from the input node. Crossover dip results from the arrival, at the input node, of this reflected signal at a point in time after the original signal falltime amplitude has dipped below a desired value. The reflected signal then belatedly reinforces the falling amplitude signal, resulting in a non-flat signal response.
In addition to these problems, prior art probes exhibit certain undesirable characteristics specific to the apparatus used to implement the probe, when used over a wide frequency range. Two types of passive probes are representative of the prior art: a low impedance probe using standard (non-lossy) coaxial cable, and a relatively high impedance probe using lossy line coaxial cable.
The first type of prior art probe typically uses standard, low loss coaxial cable with a single resistor ("tip resistor") connected to the input end of the cable. A terminating resistor with a value equal to the characteristic impedance of the cable is connected to the cable output end. The input impedance of the probe is the sum of the tip resistor and the characteristic impedance of the cable, typically, 500 ohms for a probe with an attenuation factor of 10 ("10X probe") using 50 ohm coaxial cable. The characteristic impedance of the cable used is generally a maximum of 100 ohms, and therefore, a 10X probe using 100 ohm cable typically has only a 1K ohm input impedance. This type of probe maintains this input impedance at high frequencies; however, because this impedance is relatively low, the application of this probe is limited to probing low impedance circuits.
A second type of prior art probe utilizes lossy coaxial cable. Although this type of probe has a high impedance at DC, its effective bandwidth is limited due to the lossy nature of the cable and its high input capacitance. The bandwidth of a typical lossy cable probe is 200 megahertz (Mhz), with an upper limit of approximately 500 Mhz. A second limitation of this type of cable is low input impedance. The input capacitance, which is generally in the range of 6 picofarads (pF) to 10 pF provides only 31 ohms to 53 ohms of capacitive reactance in parallel with the high input resistance. The result is low input impedance at high frequencies. In addition, the lossy cable is relatively expensive as compared to standard coaxial cable.
The above problems with prior art probes are compounded by stray tip capacitance resulting from plate effect capacitance due to the capacitive interaction between the probe tip components and the grounded shield typically encompassing the tip network section of a typical probe. This stray capacitance further reduces the input impedance of the relatively low input resistance.
3. Solution to the Problem
The apparatus of the present invention solves the above problems by providing a passive, high impedance probe which has a relatively flat frequency response over a wide bandwidth. The present invention has several novel features, each of which provides an improvement over the prior art. One novel feature of the invention is a low capacitance probe tip designed to minimize stray probe tip capacitance. Another novel feature is the use of a front-end (tip) resistor in series with a conventional RC tip network. This front-end resistor provides two functions: First, it establishes a minimum input impedance for the probe input, and secondly, it provides approximately 80% of the high frequency attenuation when working into the cable characteristic impedance.
Yet another novel feature of the invention is the use of a standard, non-lossy coaxial cable. The use of standard coaxial cable is made possible because of the high frequency attenuation provided by the tip resistor, thus obviating the need for lossy cable. Any length of cable can be used, as long as the AC terminating capacitance is approximately twice the cable capacitance, and the cable length is properly selected. When the cable length is selected in accordance with the present invention, the crossover dip and related transmission line signal distortion effects are minimized. These signal distortion effects include "overshoot," which is the result of signal voltage increasing very rapidly to substantially the voltage applied to the input of the probe, which voltage is much higher than the desired attenuated voltage at the output end of the cable. The use of standard coaxial cable provides a significantly less expensive alternative to lossy cable.
In addition, the terminating network of the present invention contains two resistors in series with the terminating capacitor which add together to provide an impedance match with the cable. These resistors are split to provide the additional 20% of the high frequency attenuation when the signal output is taken from their common node.